1. Field of the Invention
The present invention relates to a sensor control apparatus which controls a gas sensor including a sensor element whose sensor output signal changes in accordance with the concentration of a gas, and a gas detection system which includes the gas sensor and the sensor control apparatus.
2. Description of the Related Art
Conventionally, a gas detection system has been known including a sensor control apparatus which controls a gas sensor. The gas sensor has a sensor element which includes a first cell whose sensor output signal changes in accordance with the concentration of a gas.
As an example, the first cell is composed of a solid electrolyte body and a pair of electrodes, generates an electromotive force corresponding to the concentration of a specific gas (e.g., oxygen) contained in a gas under measurement (hereinafter also referred to as an “object gas”), and outputs a sensor output signal corresponding thereto.
Examples of a gas sensor including such a first cell are a linear lambda sensor which detects the concentration of oxygen contained in the object gas and an NO sensor which detects the concentration of NO contained in the object gas.
The sensor control apparatus controls operation of the sensor element such that a sensor output signal can be output therefrom, and reads the sensor output signal of the sensor element, to thereby detect the concentration of the specific gas on the basis of the sensor output signal.
Such a sensor element (first cell) may have the property of an impedance which changes with a change in its temperature (element temperature).
A sensor control apparatus which controls a gas sensor including such a sensor element operates as follows. The sensor control apparatus first supplies to the sensor element (the first cell) an impedance detection signal for detecting the impedance of the first cell, and reads a response signal output in response to the impedance detection signal. The sensor control apparatus can detect the impedance of the first cell on the basis of the read response signal, thereby detecting the element temperature of the sensor element.
As is generally known, a filter circuit having a frequency band suitable for a signal to be detected is provided in such a sensor control apparatus so as to remove high-frequency noise, to thereby ensure accurate detection of the signal.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2008-008667
3. Problems to be Solved by the Invention
However, in a sensor control apparatus configured so as to read a sensor output signal or a response signal output from the above-mentioned first cell, the detection accuracy of either one of these signals may be compromised.
Namely, since the frequency band of the sensor output signal and the response signal differ from each other, use of a filter circuit suitable for the frequency band of the sensor output signal may attenuate the response signal, thereby decreasing the detection accuracy of the response signal. In contrast, use of a filter circuit suitable for the frequency band of the response signal may superimpose noise on the sensor output signal, thereby decreasing the detection accuracy of the sensor output signal.